1. Field of the Invention
This invention relates to improvements in an optical head wherein a beam of light from a light source is focused and irradiated upon an information record face of an optical disk or the like and reflected light from the information record face is received by a light receiving element to read information of the information record face.
2. Description of the Prior Art
An exemplary one of conventional optical heads of the type mentioned is first described with reference to FIG. 5. The optical head shown includes a light source 1 such as a semiconductor laser emitting element, a collimator lens 2 for collimating light from the light source 1 into substantially parallel light, a separating element 3 such as a half mirror or a polarizing separating membrane for reflecting light from the light source 1 but transmitting reflected light from an information record medium 5 such as an optical disk, an objective lens 4 for focusing light reflected by the separating element 3 upon a face of the information record medium 5, a cylindrical lens 6 having a focusing function only in one direction, and a light receiving element 7 for receiving light focused by the cylindrical lens 6 and converting the received light into an electric signal.
In the optical head, light emitted from the light source 1 is substantially collimated by the collimator lens 2, then reflected by the separating element 3, and then focused into a dot upon an information record face of the information record medium 5 by the objective lens 4. Then, the light reflected by the information record face passes through the objective lens 4 to make substantially parallel light, and then passes through the separating element 3, whereafter it passes through the cylindrical lens 6 so that it is focuses into a linear beam of light on a light receiving face of the light receiving element 7.
The light receiving face of the light receiving element 7 is divided into such four sectoral sections as shown in FIG. 6. Thus, if light irradiated upon the information record face of the information record medium 5 comes out of a focused condition or the focus thereof is displaced radially outwardly in a direction, then the amounts of light received at the four light receiving sections will be different from each other. Making use of this fact, a focusing servo signal and/or a tracking servo signal are obtained.
With the optical head having such a construction as described above, the cylindrical lens 6 having a cylindrical surface is necessitated. However, it is disadvantageous that an operation of polishing a face of the lens into a cylindrical face is cumbersome, and upon assembly, adjustment for establishing alignment between the center and an optical axis of the lens with each other is required.
An astigmatism generating device for eliminating such drawbacks as described above is disclosed in Japanese Utility Model Laid-Open No. 63-58316. Astigmatisim is a focus defect in which electrons in different axial planes come to focus at different points.
The astigmatism generating device is constituted such that a separating element in the form of a prism which corresponds to the separating prism 3 described above has, in addition to a reflecting/transmitting face which reflects light from a light source but transmits reflected light from an information record face of an information record medium, a reflecting face for reflecting such reflected light from the information record face, and the angle of the reflecting face is inclined with respect to the reflecting/transmitting face such that the astigmatism may be reduced to zero.
With the astigmatism generating device, since an astigmatism is generated due to the angle between the reflecting/transmitting face and the reflecting face of the separating element, if the angle is set in error, then the astigmatism will be erroneous, and a necessary image will not be obtained on a light receiving element.
Further, since the linear distance from the information record face of the information record medium to the light receiving element is not reduced, it is disadvantageous that the optical head cannot be reduced in overall size nor in overall weight.